The present invention relates to the transmission of digital information and has particular application to cryptographic transmission systems.
Techniques for cryptographic transmission of digital information can be categorized as being either cryptographically self-synchronizing or non-self-synchronizing. Self-synchronizing techniques, such as the cipher text auto key (CTAK) mode of the well-known Data Encryption Standard (DES), have the advantage that the crypto-state vector generators in the encryptor and decryptor will synchronize, i.e., automatically come to store the same crypto-state vector value, without having to be initialized to the same value. Disadvantageously, however, self-synchronizing techniques extend transmission errors, meaning that a single transmission channel error will give rise to an entire burst of errors at the receiver. In high error rate environments, this can render use of encryption impractical.
By contrast, the non-self-synchronizing techniques such as key auto key (KAK) or the state-sequence-driven mode of the DES, do not extend errors. They do, however, require that a synchronization signal be transmitted when the encryptor and decryptor are to be initially synchronized, and, thereafter, whenever they become unsynchronized. This is undesirable from both the cryptanalytic and transmission efficiency points of view.